This invention relates to the biological treatment of wastewater, and more particularly to the treatment of wastewater by use of a rotating trickling filter.
Biological wastewater treatment is commonly employed as a secondary treatment following a primary treatment which usually involves simple course screening and plain sedimentation to remove the settleable solids from the wastewater. In biological treatment, growths of biological slimes or flocks are employed which utilize the pollutants in the presettled wastewater for their natural growth processes. Biological secondary wastewater treatment commonly utilizes aerobic processes for organic removal. Successful biological wastewater treatment processes provide means for bringing wastewater into contact with the biological slimes or flocks so as to supply nutrients and to also bring dissolved oxygen, usually supplied by air, into contact with the slimes or flocks adequate for their respiration.
Presently used biological treatment processes include the activated sludge process, trickling filtration, and rotating biological contactors. In the activated sludge process, including its variations such as modified aeration and step aeration, wastewater is aerated in the presence of controlled amounts of free-floating biological flocks. The air used to agitate the wastewater and flock mixture to bring about contact between the wastewater and the biological flocks also supplies the oxygen for respiration.
In the trickling filtration process, presettled wastewater is passed over slimes grown on and attached to stationary surfaces. In their basic forms the surfaces are usually rock beds but more recently formed plastic media has been used. Examples of such trickling filter media are shown in U.S. Pat. No. 3,403,095 issued Sept. 24, 1968 to Chipperfield et al and U.S. Pat. No. 3,260,511 issued July 12, 1966 to Greer.
The rotating biological contactor process is the most recently commercialized process using biological treatment. It employs rotating surfaces which are partially submerged in wastewater held in a tank and rotated to expose the surfaces consecutively to wastewater and the overlying atmosphere. The rotating surfaces can be simple spaced discs along a shaft (see British Pat. No. 935,162 to Hartmann, published Aug. 28, 1963; or U.S. Pat. No. 3,335,081 issued to El-Naggar on Aug. 8, 1967) or may take a more complex form of generally cylindrical contactor media exhibiting a greater surface area than is available by spaced discs (see U.S. Pat. No. 3,827,559 issued Aug. 6, 1974 to Gass et al). The rotating contactor is typically submerged in wastewater to about 40% of its diameter; that is, to near its axis of rotation.
Another form of rotating contactor uses a cylinder formed of screening and containing hollow plastic spheres (see Swedish Pat. No. 370,635, published Oct. 28, 1974). This latter arrangement discloses irrigation tubes mounted along the inner surface of the screen cylinder and which lift water and pour it over the plastic spheres as the drum rotates.
A hybrid process described by the inventor as a rotating activated sludge process is disclosed in Swedish Pat. No. 100,517, published Dec. 17, 1940. In that patent growth bodies such as pieces of wood or cork are tumbled within a drum into which wastewater and air are fed. In one version the growth bodies are mounted within a screen-like cylinder which in turn is mounted within an impervious cylindrical shell. Provision is made to rotate the outer shell and the cylinder separately, and lifting bodies are connected to the inner periphery of the drum in the space between the drum and the inner cylinders to carry water upwardly over the cylinder for dumping as the drum rotates.
By my invention I combine operating characteristics of both the trickling filter and rotating biological contactor to achieve what I call a rotating trickling filter. The filter material or media is packed within a hollow cylindrical shell and wastewater is introduced into one end of the shell, with the amount of wastewater controlled so that it occupies only a very small portion of the volume within the shell. The shell is rotated and the wastewater which naturally occupies the lower portion of the shell is washed over the media surfaces. The rotating trickling filter of my invention has the advantages inherent in stationary trickling filter processes and requires lower power to rotate the unit than would a rotating biological contactor assembly of comparable size because there is less hydraulic drag. There is a much lower stress on the media because the media can be supported and driven at its periphery rather than through a central shaft or tie rods which localize stress. Full contact of the wastewater within the shell with media surfaces is insured because of the low wastewater volume being treated at any particular time in the process. Rotation overcomes the plugging problems which are inherent in a stationary trickling filter.